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English definition of “air sac”

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air sac

noun [C] uk   specialized us  
a space filled with air that continues from a bird's lungs into one of its bones or another body area, used in breathing a wide part like a bag in the tubes an insect uses for breathing an alveolus (= a very small air bag in the lungs)
(Definition of air sac from the Cambridge Advanced Learners Dictionary & Thesaurus © Cambridge University Press)
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